The present invention relates to passive infra-red motion detection devices and is particularly directed to optical arrangements for achieving a wide field of view.
Infrared motion detection devices are commonly used in automatic light switches and security systems to rum on a light or to activate some other form of alarm or warning indicator when a person or motor vehicle enters a monitored area. Such devices may be used in residential lighting, for example, to illuminate a walkway as a person approaches the front door or to illuminate a driveway as a car approaches. They are also popular as energy saving devices in large office buildings or industrial plants, which may have hundreds of rooms to be illuminated day and night. The motion detection devices can save considerable energy and cost by automatically extinguishing the lights in unoccupied rooms.
The devices function by sensing heat, in the form of infra-red radiation, emanating from a person or similarly warm object as the person or object enters or moves about in the field of view of the device. An arrangement of mirrors and/or lenses directs the incident infra-red radiation to a sensor assembly that may include one or more sensors. When the sensor assembly detects an appropriate heat impulse, the device provides an electrical signal to activate the light or other alarm. A common configuration includes an array of individual lenslets, in which each lenslet focuses infra-red radiation from a particular spatial zone of limited extent to one or more sensors. The field of view of the motion detector is the region of space spanned by the totality of these zones. It is by means of these zones that the motion detector is able to detect motion. In a simple embodiment the motion detector turns on a light or provides other indication whenever a person or other warm object enters a single zone. In more complex embodiments the motion detector can be configured to turn on the light only when the warm object sequentially enters and exits a specified number of zones. In this way the motion detector detects movement within its field of view by detecting the heat from the moving object as it enters and exits one or more individual zones in the field of view. The light is typically turned off automatically if no movement is detected within a specified time interval.
Motion detectors for monitoring a full 360.degree. field of view present their own characteristic problems. The passive infrared (PIR) sensor found in popular integrated-circuit (IC) packages does not lend itself well to achieving wide fields of view. These integrated-circuit sensor packages typically include one or more planar sensing elements which are irradiated through a window on the surface of the IC package. These IC sensor packages are most sensitive to head-on radiation, incident at 90.degree. to the window surface. Their sensitivity drops off the more the incident radiation approaches the sensor from the side, i.e., the more the radiation approaches the sensor at a glancing angle. To achieve a wide field of view, known motion detectors generally require complicated optical arrangements for directing the infrared radiation from the outlying reaches of the desired field of view into the significantly narrower angular reach of the sensor sensitivity. Some motion detectors have achieved wider fields of view by compounding a plurality of integrated-circuit sensor packages, each having its own limited field of view, with optical arrangements that bring the incident radiation to the proper sensor at the proper angle to be perceived. In any event motion detectors with wide fields of view have generally involved a tradeoff among increases in cost, complexity, and the physical size of the motion detector unit, and a compromise in performance. For motion detectors approaching a full 360.degree. field of view the tradeoff is all the more stringent.